In order to ascertain whether DNA:anti-DNA immune complexes are present in the blood of patients with glomerular and inflammatory diseases other than systemic lupus erythematosus (SLE), cryoprecipitable immune complexes (cryoglobulins) have been isolated from the serum of patients with glomerulonephritis, essential cryoglobulinemia, and infections, and then compared with cryoglobulins from patients with SLE. Using a variety of techniques, anti-DNA antibodies can be demonstrated in the majority of the cryoglobulins. Similarly, employing both immunochemical and fluorochemical techniques, DNA can be demonstrated to be present in these cryoprecipitates. The data suggest that the majority of DNA present in the precipitates is double-stranded DNA. Both the DNA and anti-DNA appear to be bound in the cryoglobulins and react specifically with each other, suggesting that they are present in the precipitates as immune complexes. Since DNA:anti-DNA complexes are believed to play a role in the pathogenesis of glomerulonephritis in SLE, their presence in other inflammatory disease states suggests that they may participate in the pathogenesis of glomerulonephritis in a variety of different diseases. We propose further studies of the physiochemical and immunologic characteristics of these complexes in an effort to clarify their pathogenicity. Further studies will be carried out to determine the molecular weight(s) and complement activating characteristics of DNA:anti-DNA complexes from these various states. The results will be compared with those from complexes isolated from patients with SLE glomerulonephritis.